Role of the surface carbohydrates in sperm–egg interaction in Ciona intestinalis

Abstract

In the ascidian Ciona intestinalis, species-specific recognition and binding between spermatozoa and eggs occurs at the outer surface of the vitelline coat (the chorion). Attachment of spermatozoa to the chorion occurs via very thin, Ruthenium red-positive fibrils connecting the plasma membrane of the tip of the spermatozoon with the fibrillar tufts of the outer surface of the chorion¹. Experiments with fluorescein-conjugated lectins suggest that glucose or mannose and fucose residues are exposed at the surface both of the chorion and of the spermatozoon, while N-acetyl-glucosamine is only present at the surface of the chorion². Work presently underway shows that the prevailing sugar components of the chorion are fucose, glucose and N-acetyl-galactosamine. However, this last sugar does not seem to be exposed at the surface of the chorion². The role of the sugar residues exposed at the surfaces of the chorion and spermatozoon hi gamete recognition and binding has been investigated to determine whether these sugars are functional components of the gamete receptors. It would be expected that binding of the spermatozoa to the chorion and fertilisation should be competitively inhibited in the presence of excess fucose, glucose and/or mannose, while N-acetyl-galactosamine and N-acetyl-glucosamine should not be effective. Our present results suggest that fucose residues play a key part in the binding reaction and in fertilisation.